The requirements of rapid construction and the high labor cost of on-site construction have led to increasing use of prefabricated panels or slabs.
Concrete sandwich panels have proved successful for such purposes. The panel can include a pair of concrete plates sandwiching therebetween a foamed insulating layer made of a resinous material and held together by anchors embedded in the plates and traversing the intermediate layer.
I have described in my German Printed Application (Offenlegungsschrift) DT-OS 2,008,402 and in U.S. Pat. No. 3,757,482 the use of a tubular anchor.
This anchor is in the form of a cylinder or a sleeve embedded in both of the concrete plates and passing through the foamed resin cushion. At either end of this tubular anchor, at least now row of openings is provided and opposite pairs of these openings are traversed by a reinforcing or anchoring metal (steel) bar which is embedded in the concrete mass.
These tubular anchors have the rigidity characteristic of tubular formations in general. Since such anchors are relatively stiff, to accommodate limited relative mobility of the plates, only a small number of such anchors (usually only one) could be used for each panel or slab construction.
To accommodate thermal dimensional changes and like movements, while maintaining a secure interconnection of the plates, in addition to the tubular anchor, the plates are connected by auxiliary anchors.
The latter have generally been curved hairpin-like fasteners having a U-bent elbow and two tines. The bent elbow of these pins is embedded in one of the concrete plates and the tines of the pins are undulated at their ends and are embedded in the other of the concrete plates. A large number of such pins are required and the specific stress to which they are subjected is considerable, so that they tend to break.